一個頭不只兩個大

一個頭兩個大(yi2 ge5 tou2 liang3 ge5 da4)
一 is "a" or "one".
個 is a measurement for heads.
頭 is "head".
兩 is "two".
大 is "big".
It means "I have a head but it's two times of normal size".
Rather, "I have a head but it's two times of normal size, for it's full of trouble or worriment ".
That is to say, "I feel something troublesome."

一個頭不只兩個大 (yi2 ge5 tou2 bu4 zhi3 liang3 ge5 da4)
不 is "no" or "not".
只 is "only".
不只 is "more than"
It means "I have a head but it's bigger than two times of normal size"
Thus it means "I feel something so troublesome."